Pet News
Heartworm … should I be concerned?
Spring is usually the time when we test dogs for Heartworm disease in southwestern Ontario. This tradition has been going on since the early 1980's and from time to time dog owners are inclined to wonder if it really is as big a problem as veterinarians profess it to be. Here is some information about Heartworm to familiarize you with the risks and help you make informed decisions about how to manage this annual spring threat to your family member.
Heartworm is a thin white worm that can grow up to 14" in length. Light infections occupy the pulmonary artery which carries blood from the heart to the lungs. Heavier infections (over 25 worms) occupy the pulmonary artery and the right ventricle and still heavier infections (over 100 worms) will occupy the entire right side of the heart. Heartworm is spread by biting mosquitoes. Cats can get heartworm disease as can people, though neither is the usual host for this parasite. Heartworm disease leads to congestive heart failure because the physical presence of the worms in the heart interferes with the pumping of blood. The disease is treatable if diagnosed, but the treatment is as dangerous as the disease itself.
Certain conditions are required for the spread of Heartworm. These include the presence of the types of mosquitoes capable of spreading Heartworm, warm enough temperatures and the presence of infected dogs or coyotes in the area. Starting with an infected dog or coyote, there must be both male and female Heartworms to breed and produce baby Heartworms (larvae). These baby Heartworm larvae are called microfilaria and they swim throughout the infected animal's blood stream. These first stage (L1) microfilaria can live up to two years and if they aren't picked up by a biting mosquito in this time, they die.
The mosquitoe biting the infected canine picks up the L1 microfilaria and if the temperature remains above 14 degrees C the L1 matures to an L2 and then an L3 microfilaria in a few weeks. The warmer the temperature, the shorter the maturation phase. Our mosquito infected with L3's now bites another dog/coyote and the L3 matures to an L4, then an L5 and finally an adult worm. The mosquito biting the dog doesn't infect the dog by depositing those L3's directly into the dog's bloodstream. Rather, the mosquito leaves a little blob of mosquito saliva containing L3's. Provided there is enough humidity and the blob of mosquito spit doesn't dry out too quickly, the L3 swims through the spit, through the hole in the skin left by that biting mosquito and into the dog's skin where it stays for a week or two before maturing into an L4. The L4 remains in the skin for another 3 months, matures into an L5, enters the dog's blood stream and heads for that pulmonary artery as it matures into an adult worm.
The products we use to protect our canine family members act differently. All products will kill the L3 and L4 stages in the skin. One of the products also kills the L5 stage. None of the products will kill adult worms. In order to kill this stage it is necessary to administer a toxic arsenic-based compound intravenously. This kills the worms living in the heart and the pulmonary artery. Unfortunately, the dead worms don't just disappear and as the heart continues to pump, they dislodge from the heart, break into smaller pieces and circulate throughout the body where they can lodge in the lungs, the brain, the liver, the kidneys and other organs where they cause further troubles.
Recently we reviewed statistics for heartworm disease to see if it was still a problem in the London area. One of the laboratories that tests for the disease reported that in 2008 there were 25 positive cases of heartworm in London and a total of 1023 positive cases in Ontario. St. Thomas has 9 positive cases, Arva had 3 and Strathroy had 24. As these results are from only one laboratory, it is safe to conclude that there are more positive cases which were not reported in this survey.
It would appear that heartworm disease remains a threat in the London area even after testing for this disease and using preventatives for about 30 years. Heartworm testing isn't for everybody and some dog owners prefer to roll the dice and hope their dog won't get bitten by an infective mosquito. For other dog owners, taking such a passive approach is out of the question.
In order to test for Heartworm disease it is necessary to collect a small volume of blood. Some veterinarians test for the disease using a test kit in their own practice; others send the blood to a laboratory for testing. Some Heartworm tests check for L1 microfilaria while others detect the presence of adult worms. None of the tests can find L4, or L5 microfilaria. If the test is positive for microfilaria it means that dog has reproducing adult worms. However a negative test for microfilaria doesn't rule out the possibility of adult worms because the dog could still have adults of the same sex. They won't be reproducing. We call this Occult Heartworm infection. That's why many veterinarians start with the test for the adults.
Heartworm preventatives come in two main forms; a topical that is applied directly to the skin usually between the shoulder blades so it can't be licked off, or a chewable treat. All products are given once a month. Depending on the product, a Heartworm preventative might also help in the treatment and prevention of fleas, ticks, ear mites and intestinal parasites or worms. Remember that all products will kill the L3 and L4 stages, well prolonged use of some of these preventatives also kill the L1 stages, remembering that L2 and L3 are in the mosquito. So if a dog already infected with adult heartworms is put on one of these preventatives, the microfilaria test will be negative, but the adult test will still be positive. That's why so many veterinarians are hesitant to simply put dogs on preventatives year after year without testing first for the adults.
In the end, everyone must decide whether or not they want to have their family member tested and put on a preventative medication. We thought it might prove helpful to know some of the facts about Heartworm disease before making your decision.
